linux-kernel - Re: [PATCH RFC/TEST] sched: make sync affine wakeups work

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Date:	Sun, 4 May 2014 17:14:59 +0530
From:	Preeti Murthy <preeti.lkml@...il.com>
To:	Rik van Riel <riel@...hat.com>, umgwanakikbuti@...il.com
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	george.mccollister@...il.com, ktkhai@...allels.com,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Subject: Re: [PATCH RFC/TEST] sched: make sync affine wakeups work

Hi Rik, Mike

On Fri, May 2, 2014 at 12:00 PM, Rik van Riel <riel@...hat.com> wrote:
> On 05/02/2014 02:13 AM, Mike Galbraith wrote:
>> On Fri, 2014-05-02 at 00:42 -0400, Rik van Riel wrote:
>>
>>> Whether or not this is the right thing to do remains to be seen,
>>> but it does allow us to verify whether or not the wake_affine
>>> strategy of always doing affine wakeups and only disabling them
>>> in a specific circumstance is sound, or needs rethinking...
>>
>> Yes, it needs rethinking.
>>
>> I know why you want to try this, yes, select_idle_sibling() is very much
>> a two faced little bitch.
>
> My biggest problem with select_idle_sibling and wake_affine in
> general is that it will override NUMA placement, even when
> processes only wake each other up infrequently...

As far as my understanding goes, the logic in select_task_rq_fair()
does wake_affine() or calls select_idle_sibling() only at those
levels of sched domains where the flag SD_WAKE_AFFINE is set.
This flag is not set at the numa domain and hence they will not be
balancing across numa nodes. So I don't understand how
*these functions* are affecting NUMA placements.

The wake_affine() and select_idle_sibling() will shuttle tasks
within a NUMA node as far as I can see.i.e. if the cpu that the task
previously ran on and the waker cpu belong to the same node.
Else they are not called.

If the prev_cpu and the waker cpu are on different NUMA nodes
then naturally the tasks will get shuttled across NUMA nodes but
the culprits are the find_idlest* functions.
   They do a top-down search for the idlest group and cpu, starting
at the NUMA domain *attached to the waker and not the prev_cpu*.
This means that the task will end up on a different NUMA node.
Looks to me that the problem lies here and not in the wake_affine()
and select_idle_siblings().

Regards
Preeti U Murthy

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